1. Field of the Invention
This invention relates generally to apparatus and methods for measuring angular displacement. More particularly, this invention relates to measuring the angle of attack of a missile seeker without mechanically attaching external devices to the portion of the seeker head that is movable with respect to the missile frame.
2. Description of the Prior Art
The angle of attack of a missile is the angle between the longitudinal axis of the missile and the direction the missile is traveling. Ordinarily the angle of attack should be as small as possible to reduce wind drag, provide greater range and provide a greater impact velocity when the missile reaches its target. A seeker mounted in the missile has a portion that is rotatable in three dimensions relative to the frame. As the seeker moves toward the target, the angle of attack may be measured by measuring the angle between the movable section and the missile frame.
Previous attempts to measure the angle of attack of a seeker have included mounting rotational potentiometers on the axles of the movable section. Other attempts to make this measurement have included mounting linear potentiometers between the seeker and the frame of the missile. These techniques have failed because they induced errors in the operation of the seeker. Such devices have the undesirable effect of changing the natural frequency of oscillation of the seeker. The natural frequency should be known to verify flight characteristics.
U.S. Pat. No. 4,790,493 discloses a rate gyro in a seeker head of a missile. The rate gyro is stimulated to nutate with its natural nutation frequency in inertial space for scanning a field of view. The roll rate or roll angle may be determined by processing the difference of the rotational frequency and the nutation frequency of the rate gyro relative to the missile.
U.S. Pat. No. 4,830,311 discloses a guidance system in which a homing head or seeker is mounted on-board a missile. The guidance system allows establishment of inertial references via information derived from a seeker that need not be isolated from the missile body. The seeker has a range measuring function which may be used in combination with accelerometers to control the pitch, yaw and roll stabilization of the missile.
U.S. Pat. No. 4,791,573 discloses a system for determining deviations in the state of motion of a projectile from an intended state of motion. The system includes a comparison module that receives the outputs of a sensor array. The comparison module converts the sensor outputs into a measurement vector and computes the deviation of this measurement vector from an intended measurement vector received from a control system. The comparison module then determines the difference between this measured deviation and the deviation predicted by a Kalman filter.
U.S. Pat. No. 4,699,333 discloses an on-board flight control system for controlling pitch, yaw and roll. The system has a plurality of control panels operated by an actuator drive. The edge of the control panels is slanted so that when the panels are in an open position, clockwise and counterclockwise roll of the missile can be controlled.
U.S. Pat. No. 4,676,456 is directed to a roll reference for a strap down seeker in a spinning projectile. The reference is obtained by (a) determining the frequency spectrum of signals out of an accelerometer that is sensitive to roll precession and nutation forces, (b) separating the signals indicative of the roll forces, and (c) processing the separated roll force signals to determine the roll reference.
U.S. Pat. No. 4,646,990 is directed to a magnetic roll sensor calibrator. A coil mounted inside a spinning guided missile has an electrical current induced by interaction with the earth's magnetic field. A similar coil mounted on a launch platform spins at the same rate as the coil inside the spinning object. A phase signal is generated for the launch platform coil to provide a vertical reference that is used to correct guidance commands provided to the coil in the missile. A hold fire indicator is provided to inform the operator when the output from the launch platform's coil is above or below a predetermined level sufficient for adequate roll angle compensation.
U.S. Pat. No. 4,637,571 discloses an optical guidance system in which a body fixed electronic image stabilization of television imaging is used to allow strapdown seeker guidance in a missile. The body fixed electronic image stabilization compensates for routine vibrational and rotational motion experienced by a missile in flight. Compensation is accomplished by deliberately underscanning the camera and driving the camera's deflection coils with signals from pitch and yaw body rate sensors on the missile. The image developed on the camera detector raster is moved in an equal and opposite direction to the sensed motion as the motion occurs. Compensation thus stabilizes the resultant image, which should otherwise be a blur of motion on the display screen.
U.S. Pat. No. 4,624,424 discloses an on-board pitch, yaw and roll control actuator system for a missile. A plurality of control panels operated by an actuator drive are positioned in the airstream of the missile for controlling the direction, orientation and speed of the missile.